Angiogenesis, the process by which new blood vessels are formed, is a fundamental process underlying many aspects of vertebrate growth and development including embryogenesis and fetal development. It is also crucial in ongoing physiological responses such as wound healing.
Angiogenesis is regulated by a complex combination of angiogenesis-stimulating growth factors and angiogenesis inhibitors. The balance between pro- and anti-angiogenic modulators governs when, where and to what extent angiogenesis occurs depending on the developmental state and/or physiological state of the organism. Misregulation of angiogenesis, leading to either excessive or insufficient vessel formation, can have significant consequences and is associated with a variety of pathological conditions. Excessive angiogenesis is associated with, for example, conditions such as cancer, chronic inflammatory conditions, ocular disorders and cardiovascular diseases. Similarly, inadequate angiogenesis is implicated in, for example, ischemic chronic wounds and some infertility.
In cancer, angiogenesis is crucial for the development of many cancers, for tumour growth and for metastasis. Tumours promote angiogenesis by the secretion of growth factors such as VEGF inducing blood vessel growth into the tumour. Blood vessel development provides the tumour with the required supply of nutrients and oxygen and a pathway for the elimination of waste products. Increased tumour vasculature then provides increased possibility for the tumour to metastasize. Anti-angiogenic therapy has recently emerged as a promising avenue for the treatment of cancer and may offer advantages over more traditional anti-cancer therapies, for example, the possibility of reduced susceptibility to the development of resistance. However, despite promise, there has to date been limited success in the development of efficacious anti-angiogenic agents and there remains a need for the identification of new targets and therapeutic molecules.
In view of the central role of normally regulated angiogenesis in development and numerous physiological processes, allied with the significant clinical consequences of abnormal angiogenesis (both up- and down-regulated), there is a clear need for the development of novel therapeutic options for, where required, the promotion of angiogenesis and for the inhibition of angiogenesis.
MicroRNAs (miRNAs) are an abundant class of highly conserved, small (typically 21-23 nucleotides) endogenous non-coding RNA molecules. miRNAs serve as post-transcriptional regulators of gene expression. They are crucial to many normal cellular functions, and play critical roles in, for example, cellular proliferation and differentiation, embryonic development, inflammation, immunity and many metabolic processes. Specific miRNAs, including expression patterns and altered regulation of expression of individual miRNAs, are also increasingly being implicated in a variety of disease conditions, including cancer and cardiovascular disease.
More than 1000 miRNAs have been identified to date, and more than 400 miRNAs with known sequence have been found in humans (see for example, http://microrna.sangerac.uk/sequences/index.shtml). Individual miRNA typically bind incompletely to their cognate target messenger RNA (mRNA) and as such each miRNA may bind to, and potentially regulate, many target mRNAs. Computational analysis suggests that there may be several hundred mRNA targets for any given miRNA. Accordingly, a unique miRNA may regulate the expression of one or more (potentially hundreds) different genes.
Mature miRNAs are derived from so-called pri-miRNAs that are transcribed from regions of non-coding DNA. Pri-miRNAs, usually containing several hundred nucleotides, are processed into stem-loop precursors (pre-miRNAs) of approximately 70 nucleotides by RNase III endonuclease. Pre-miRNAs are actively transported into the cytoplasm where they are further processed into short RNA duplexes, typically of 21-23 bp. The functional miRNA strand dissociates from its complementary non-functional strand and locates within the RNA-induced-silencing-complex (RISC). (Alternatively, RISC can directly load pre-miRNA hairpin structures.) miRNAs bind the 3′UTRs of target mRNAs and important in this binding is a so-called ‘seed’ region of approximately 6-7 nucleotides near the 5′ end of the miRNA (typically nucleotide positions 2 to 8). The role of the 3′ end is less clear. miRNA-induced regulation of gene expression is typically achieved by translational repression, either degrading proteins as they emerge from ribosomes or ‘freezing’ ribosomes, and/or promoting the movement of target mRNAs into sites of RNA destruction.
The present invention is predicated on the inventors' surprising finding that expression of a subset of miRNAs is downregulated during angiogenesis and that overexpression of such miRNA inhibits angiogenesis. Accordingly, the present invention opens avenues for the promotion or inhibition of angiogenesis and novel therapeutic approaches to the treatment of conditions associated with abnormal angiogenesis.